1. Field of the Invention
The present invention relates, in general, to a method and system for use with solder pumps. Specifically, the present invention relates to a method and system, for use with solder pumps, which provide the active introduction of flux onto one or more solder pump nozzle interfaces.
2. Description of Related Art
An integrated circuit is a device consisting of a number of connected circuit elements, such as transistors and resistors, fabricated on a single chip of silicon crystal or other semiconductor material. Integrated circuits are normally fabricated in "chips" constructed from "wafers" of semiconductor material. A "chip" is a minute piece of material cut from a single crystal on which electronic circuit active and passive elements are mounted, usually by etching, deposition, and diffusion processes, to form an integrated circuit. A "wafer" is a thin slice of a suitable semiconductor, such as a silicon or germanium crystal, on which microcircuits are constructed by using various techniques, such as diffusion, etching, and evaporative deposition of dopants. Chips are constructed from a wafer by constructing a large number of microcircuits on a wafer and then scoring and breaking the wafer into chips, also referred to as "dice." An individual chip can thus been referred to as a "die."
After fabrication, the integrated circuits are generally utilized to perform a function in some overall system (for example, Application Specific Integrated Circuits are now being utilized to perform many specialized features in network server computers). In order to function with other circuit components in such an overall system, the integrated circuits must communicate with those other circuit components. Such communication is typically achieved via conducting paths laid out on what is known as a printed circuit board.
A printed circuit board is typically a flat board made of material on which integrated circuit packages are mounted, typically by either what is known as surface-mount technology, or by what is known as pin-through-hole technology. The surface-mounted electronic packages are connected electrically, and thus communicate, by predefined conductive metal pathways that are printed on the surface of the board. The integrated circuit packages have protruding metal leads which function as input/output (I/O) connections to the integrated circuits and which are soldered to the conductive metal pathways of the printed circuit board.
Integrated circuit packages, and in particular those utilized in digital electronic systems such as personal computers, have significant size constraints associated with them (e.g., those utilized with notebook computers). Such integrated circuits have been undergoing significant increases in functionality with successive design generations. The size constraints typically associated with integrated circuits, in combination with functionality increases, have resulted in an ever increasing number of integrated circuit devices being built into packages of constrained size.
The increasing number of integrated circuit devices being built into packages of constrained size has resulted in a correspondingly greater number of I/O connections to and from the chip being spaced closer and closer together. The associated decreases in the dimensions of integrated circuit package interconnections have had a direct effect on the wiring and interconnect patterns of printed circuit boards.
Conventional techniques for solder bonding integrated circuit device packages to printed circuit boards are either approaching their technology limits or proving to be expensive to implement with the small dimensions typical of advanced designs. The prevailing technology, stencil printing of solder paste on printed circuit boards, is at its limits while the prevailing alternative technique, forming solder deposits on printed circuit boards by plating, is relatively expensive.
An analogous situation applies to the formation of solder deposit arrays on an integrated circuit die, such as with flip-chip devices, or on integrated circuit packages. The reflow connection of solder balls or solder columns onto integrated circuit dice or packages is likewise a complex and expensive endeavor. Furthermore, the advent of flip-chip and chip-scale-packaging technologies are stretching the limits of standard surface-mount technology, and is complicating the existing infrastructure for such surface-mount technology.
Thus, there has been developed a surge of interest in dispensing or jetting molten solder directly onto integrated circuit dice (individually or in wafer form), integrated circuit packages, and printed circuit boards. There are numerous advantages to the use of a solder droplet dispensing or jetting technology when the quality of the molten solder droplets can be controlled in volume and deposit location. The ability to accurately locate the molten solder droplets as deposited on a substrate has proved to be a manageable task. However, the realization of a molten solder dispensing system which is reliable enough for a manufacturing application, to the extent that the molten solder droplets are consistent in volume and formation has yet to be fully realized.
One highly innovative advance in the area was the International Business Machines Corporation's Electrodynamic Pump for Dispensing Molten Solder (disclosed in U.S. Pat. No. 5,377,961 which is hereby incorporated by reference in its entirety). In general, this pump uses a programmable current source to place an electric current through the liquid solder as it is flowing through a conduit. Additionally, a magnet or magnetic coil is disposed adjacent the conduit in order to provide a magnetic field in the same plane as the electric current. The conduit supplies liquid solder to a nozzle which then deposits a droplet of solder onto a printed circuit board, or the like. The magnetic coil and current source are disposed such that the plane in which the magnetic field and current vector lie is perpendicular to the axial direction of the conduit with the liquid solder therein. Therefore, as the current is applied, in a first direction through the solder, a force is exerted on the solder in a direction consistent with the "right hand rule". Conversely, if the direction of the current is reversed, then the direction of the force exerted on the solder is also reversed. Therefore, those skilled in the art will appreciate how the present invention is capable of substantially instantaneously reversing the force exerted on the solder, by selectively alternating the current therethrough. Further, it can be seen that by reversing the force exerted on the solder, immediately subsequent to applying a force exerted on the solder to urge it outwardly from the nozzle, an extremely small droplet of solder can be dispensed from the nozzle and deposited on a PCB, or the like.
Though early tests established the viability of the electrodynamic pump for dispensing molten solder droplets, evaluation in a manufacturing setting, where repeatability of droplet dispensing, and consistency and stability of droplet size are crucial, indicated that refinements were needed. In this regard, testing determined that the molten solder located in close proximity to the nozzle aperture did not remain a homogeneous mixture of pure solder with time. Metallurgical analysis of solder ejection nozzle residues and camera analyses of droplet dispensing confirmed that there was a mixture of pure solder and oxidized tin and lead within the nozzle which disrupted the dispensing dynamics, with such combination resulting in the creation of droplets with non-uniform physical properties such as size and volume.
Testing of the electrodynamic solder pump determined that the inner surfaces of the nozzle were dramatically different after a few hours of operation, notwithstanding the consistency of the input parameters which define the pressure of pulses creating the single shot droplets. Further investigation confirmed that the instability was attributable to the non-homogeneous nature of the solder near the nozzle aperture, the non-homogeneous solder being composed of the earlier noted mix of pure solder with oxidized tin and lead (dross).
As a result of the foregoing testing, it was found that in order to achieve the desired small perfectly-formed drops of molten solder, it is essential that the dynamics of formation of the solder droplets be consistent and repeatable, and that the molten solder column inside the pump be free to move with very little viscous drag. It has been found that one of the critical factors in achieving such uniformity, consistency of droplet formation, and such low viscous drag is the keeping of the solder nozzle (or tip) free from dross (a term used to refer to impurities arising from oxidation and/or contamination), which has been found empirically to give rise to irregular, non-repeatable dispensing.
The preferred method of keeping the solder nozzle free from dross is to apply what is known as flux (a substance applied to surfaces to reduce oxides) to the solder nozzle subsequent to the release of a solder drop. The application of such flux is for the purpose of thoroughly coating internal solder nozzle surfaces which are not wetted by molten solder, thereby inhibiting the formation of dross and the collection of other impurities upon such surfaces.
The problem with such application of flux is that present methods and systems do not ensure that the applied flux will indeed thoroughly coat internal nozzle surfaces not wetted by molten solder. Furthermore, present methods and systems of applying such flux are somewhat crude and inelegant, ranging from swabbing the solder nozzle with a swab coated with flux (which in itself can introduce impurities from such swab) to dipping the solder nozzle into some film or reservoir of flux.
Present methods and systems are all crude and inelegant in that they are essentially passive. That is, such methods and systems either place the flux in/on the nozzle, or place the nozzle in/on the flux, and depend on capillary action to pull the flux into crevices and onto critical surfaces internal to and upon the nozzle not covered by initial direct contact with the flux. In other words, as used herein, the term "passive" means that there is no mechanism other than capillary action that draws the flux into the recesses of the nozzle.
It is therefore apparent from the foregoing that a need exists for a method and system which will provide the active application of flux to critical surfaces internal to and upon molten solder nozzles.